This invention relates to multilayer structures comprising a polymeric layer and an inorganic oxide gas or vapor barrier layer such as plastic containers coated with a inorganic oxide barrier layer. More particularly, this invention relates to plastic beverage containers and enhancing the gas or vapor barrier properties of the container. Still more particularly, this invention is particularly applicable to PET/SiOx structures, such as rigid PET containers that have been coated on the exterior with a thin SiOx layer.
Polymeric materials have numerous advantages as packaging materials for food and beverages. They are lightweight, impact resistant, and easily shaped. Accordingly, they have enjoyed widespread popularity. Unlike glass and metal, however, all polymers exhibit a measurable degree of permeability to gases and vapors. This deficiency inherently limits the use of polymers in more demanding applications, especially where oxygen ingress or carbon dioxide loss affects the quality of the contained food or beverage.
Numerous technologies have been developed to decrease the permeability of polymers, and thus increase their range of applicability to food and beverage packaging. One of the most promising approaches has been the deposition of thin layers of inorganic oxides on the surface of the polymers, either prior to or after mechanically forming the polymer into the finished container. Inorganic oxides, especially silicon dioxide, have been explored extensively, because of their transparency, impermeability, chemical inertness, and compatibility with food and beverages.
Inorganic oxides can be deposited onto a polymeric surface by a number of techniques, including sputtering and various types of vapor deposition including plasma vapor deposition, plasma enhanced chemical vapor deposition, and electron beam or anodic arc evaporative vapor deposition. Although each technique has its own advantages and disadvantages, they all allow the deposition of nanometer-thick layers of the oxide onto the preformed polymer surface. Because of the thinness of the layer, the resulting structures retain most of the physical properties of the base polymer, but can exhibit reduced permeability.
Despite this, commercialization of containers based on polymeric/inorganic oxide multilayer structures has been slow, and is mostly limited to flexible containers made by post-forming coated films. In particular, rigid polymeric containers with inorganic oxide coatings have proven difficult to develop. This is because that, although the deposition of inorganic oxides onto the surface (especially the exterior surface) of a rigid container is not difficult to accomplish, heretofore those containers have not exhibited sufficient reductions in permeability over the uncoated containers. This is in spite of the fact that the inorganic oxide coating is typically deposited over the entire surface of the rigid container.
The reason for this modest decrease in permeability (permeability decrease is equivalent to barrier increase) is due to the presence of residual pinholes in the inorganic oxide layer. Pinholes are created in part by pressurization of containers, such as when containers hold carbonated beverages. The surface area occupied by these pinholes is usually quite small (on the order of less that 1% of the total surface); however, the impact of these pinholes is far greater than their surface area would suggest. This is because diffusion through a polymer occurs in all three spatial dimensions; thus, each pinhole can drain a much larger effective area of the container surface than the actual area occupied by the pinhole.
Because the surface of rigid containers is inherently less smooth than the surface of biaxially oriented films, the pinhole density on coated containers is much greater than that for films. Thus, whereas barrier improvements of 10-100xc3x97 are possible when biaxially oriented PET film is coated with silicon dioxide; barrier improvements of only 2-3xc3x97 have been obtained when rigid PET containers are similarly coated and used to hold carbonated beverages. This reduced barrier improvement is due in part to pressurization of the container. In addition, when the silicon oxide layer is on the external surface, it is subject to mechanical degradation on handling of the container, such as that which occurs in normal package filling operations.
Numerous methods have been explored to address this problem. The most common approach has been to deposit thicker layers of the oxide; however, this approach is inherently self-defeating. Thicker layers are less flexible and less extensible than thin layers, and therefore more prone to fracturing under stress. Another method is to apply multiple layers of inorganic oxides, sometimes with intermediate processing to redistribute the pinhole-causing species. This approach also has met with little success, in part because of the greater complexity of the process, and because of its modest impact on barrier improvement. A third method has been to supply an organic sub-layer on the polymer surface to planarize the surface and cover up the pinhole-causing species prior to laying down the inorganic oxide. This method also greatly increases the complexity and cost of the overall process, and similarly only affords modest improvements in barrier performance. A fourth approach has been to melt-extrude a second polymer layer on top of the inorganic oxide layer, and thus provide additional resistance to gas flow through the pinholes. Thus, Deak and Jackson (Society of Vacuum Coaters, 36th Annual Technical Conference Proceedings, 1993, p318) report than applying a 4 micron layer of poly(ethylene-co-vinyl acetate) on top of a PET/SiOx structure improved the barrier property by 3xc3x97, and applying a 23 micron top layer of PET improved the barrier performance by 7xc3x97.
Despite the barrier improvement demonstrated by Deak and Jackson, there has been little commercial implementation of this approach, for several reasons. First, melt extrusion of a second polymer onto a polymeric/inorganic oxide film imparts substantial thermal stress to the preformed structures, often severely compromising their barrier performance. Second, structures where the two polymers are different are inherently more difficult to recycle than structures composed only one polymer. Third, coextrusion of a second polymer onto preformed rigid containers is nearly impossible with current technology, and is cost prohibitive for large volume applications in the food and beverage industry.
Thus, there is a need for multilayer structures with enhanced barrier performance, especially polymeric/inorganic oxide multilayer structures such as silica coated PET containers.
This invention solves the above-described problems in the prior art by providing a coated multilayer structure comprising a polymeric base layer, an inorganic oxide gas barrier layer on the surface of the polyermic base layer, and a top coat on the inorganic oxide gas barrier layer comprising a soluble compound capable of reducing the permeability of the multilayer structure to gas or vapor. More particularly, the soluble compound has a plurality of carboxyl, hydroxyl, or carboxamide functional groups, has a melting point above room temperature (25 C), is chemically non-reactive with the inorganic barrier coating, is water soluble, and is nontoxic. The soluble compound of the top coat blocks ingress or egress of gas or vapor through the pinholes. The top coat is particularly suitable for blocking ingress or egress of oxygen and carbon dioxide.
This invention also encompasses a method for enhancing the gas or vapor barrier properties of a multilayer structure comprising a polymeric base layer and an inorganic oxide gas barrier layer on a surface of the polymeric base layer. This method comprises applying to the inorganic oxide gas barrier layer a top coat comprising the above-described soluble compound. Desirably, the soluble compound is applied to the inorganic oxide gas barrier layer in a form of a solution such as an aqueous solution. The multilayer structure is allowed to dry such that the solvent evaporates and the soluble compound remains as a top coat.
The soluble compound of the top coat can be polymeric or monomeric. Suitable polymeric compounds for the top coat include carboxymethyl cellulose, polyacrylamide, polydextrose, polyacrylic acid, and polyvinyl alcohol. Suitable monomeric compounds for the top coat include sucrose, caramel, and citric acid.
The treatment of this invention is particularly useful for enhancing the gas or vapor barrier characteristics of containers such as food or beverage containers. This invention is particularly useful for enhancing the gas or vapor barrier characteristics of packaged beverage containers such as carbonated soft drink containers. According to a particular embodiment, the top coat of this invention is applied to a silica-coated polyethylene terephthalate container.